An audio reproduction system may generally be represented by a series arrangement of a signal processor, such as a digital signal processor (DSP), followed by a power amplifier. The power amplifier may be connected to and drive a loudspeaker or in the case of stereo reproduction, a pair of loudspeakers. Such an arrangement is generally known as an audio reproduction chain. The power amplifier and the loudspeakers will have operational limitations related to the specific type of components used. For example the power amplifier may have electrical limitations such as, a maximum operating (or output) levels defined as a maximum peak output voltage or current or a maximum peak gain. The loudspeaker may have mechanical limitations such as, inter alia, a maximum membrane excursion and also thermal limitations such as a maximum voice coil temperature. The maximum output level of a specific amplifier and the maximum excursion of a specific loudspeaker may be known and may thus be used to define maximum safe operating levels of the audio reproduction system.
In audio reproduction systems a term known as headroom refers to the amount by which the signal-handling capabilities of an audio system exceed maximum operating levels. Headroom can be thought of as a safety zone allowing transient audio peaks to exceed maximum operating levels of the system without exceeding for example the maximum excursion level of the loudspeaker or a clip level of the amplifier (defined by the maximum gain of the amplifier).
In mobile devices or portable electronic devices, such as for example smart phones, tablets or the like, the operational limitations (be that mechanical or electrical) may be limited by the use of miniature components which would be required to fit within the device housing. The requirement of miniaturisation will be particularly apparent for loudspeakers, where membrane excursions will be limited by the size of the loudspeaker that can be accommodated within the device housing. In addition because such devices are generally powered by battery they may have power consumption limitations whereby any component, in particular any power amplifying component may be required to operate at low power typically 0.5 Watts per channel. Therefore, for mobile devices or portable electronic devices the issues concerning mechanical limitations and electrical limitations of the components will be particularly apparent. Amplifiers are available with the ability to maximize the audio output in terms of sound pressure level or low frequency response for a maximum electrical limitation of the amplifier itself or for a mechanical or thermal limitation of the loudspeaker it drives. Such amplifiers are known as smart amplifiers.
Increasingly mobile devices or portable electronic devices are being equipped with one or more pairs of loudspeakers thereby, in theory, allowing stereo audio reproduction. The loudspeakers will typically be located in close proximity to each other, as determined by the size of the mobile device in question. In practice however, the perceptible stereo image will be narrow or even non-existent. Stereo widening techniques may improve spatial sound reproduction, and therefore an improved stereo image, on speakers located close to each other.
Known techniques for stereo widening include techniques, such as spatial effect techniques, may rely on acoustic cross-talk cancellation. So-called transaural audio systems of the type illustrated in FIG. 1a are one of many popular methods of acoustic cross-talk cancellation. With reference to FIG. 1a, an example transaural audio system 10 may utilise a cross-talk canceller 12 in the audio reproduction chain. The cross-talk canceller 12 comprises left L and right R input channels and left and right output channels connected to respective left and right channel power amplifiers 13 followed by left and right channel loudspeakers 14. Such, transaural audio systems aim to, at least partially, cancel the acoustic cross-talk (indicated by dotted lines) between each loudspeaker and the opposite ear of a listener 15. The cross-talk canceller 12 may be implemented, for example, by the architecture shown in FIG. 1b which comprises an arrangement of filters A, B, C, D.
However, such transaural audio systems are known to be ill-conditioned because the difference between the direct path head related transfer function (HRTF) (indicated in FIG. 1a by the solid lines from the left and right side loudspeakers to the respective left and right side ears of listener 15) and the cross-talk path HRTF is very small at low frequencies, resulting in large gains in the cross-talk canceller filters low frequency response. This problem is particularly apparent when the two loudspeakers are positioned close together, such as in the case of a portable electronic or mobile device, because the distance between cross-talk paths will be small. The smaller the distance between loudspeakers the larger the required low frequency signal boost. Techniques which rely on signal boosting can result in saturation of the output signal or mechanical overdrive of the speakers.
To overcome problems of saturation the signal level must be scaled down or dynamically compressed. However scaling or compression can ultimately reduce sound pressure levels from the loudspeakers and have a negative effect on audio quality, due to for example dynamic compression artifacts. Furthermore, the amount of boost is frequency dependent, in that the signal will generally boosted more in the low frequency range than the high frequency range. Even when the stereo widening level does not saturate, it can lead to excessive loudspeaker excursion, also known as mechanical overdrive, beyond safe operating mechanical limits which can introduce audible distortions and potentially damage the membrane of the speaker.